1. Field of the Invention
The present invention relates to an active balun with a stacked structure and, more particularly, to an active balun with a stacked structure capable of compensating for an imbalance in parasitic capacitance of a common drain (CD) MOS transistor and that of a common source (CS) MOS transistor included in the stacked structure, to thereby prevent signal distortion and guarantee linearity.
2. Description of the Related Art
In general, an analog transmitter receives an in-phase (I) signal and a quadrature-phase (Q) signal which has been generated by converting a digital signal received from a modem into an analog signal by using a digital analog converter (DAC). This means that the digital signal carrying information is divided into an analog signal of a real number part and an analog signal of an imaginary number part, which are then transmitted. The transmission signals are transferred in the form of differential signals including I and Q signals to a circuit constituting a transmitter system.
Recently, in a direct conversion scheme most commonly used for the structure of the transmitter system, a differential-to-single circuit is largely disposed between an up-mixer and a drive amplifier or behind the driver amplifier, so as to be used. When the differential-to-single circuit is used between the up-mixer and the drive amplifier, the driver amplifier can be implemented as a single-ended type driver amplifier, advantageously reducing power consumption, and when the differential-to-single circuit is used behind the drive amplifier, the number of baluns used as external elements and other components can be halved to advantageously lower product unit cost.
With regard to this configuration, however, in the former case, the gain, linearity and oscillation performance of the transmitter may deteriorate, and in the latter case, the drive amplifier must be implemented as a differential type amplifier, consuming a great deal of current as compared with the former case. The differential-to-single circuit is positioned at the rear side among unit blocks constituting the transmitter system, so it must be designed to satisfy the performance required by the transmitter even if performance in areas such as linearity is degraded as mentioned above.
The related art active balun, which converts a differential signal into a single signal, has a structure in which the differential signal is converted into the single signal by circuitry within a chip as well as a structure in which the balun, an external element, is used outside a chip. In the case of the active balun having a stacked structure including a common drain MOS transistor and a common source MOS transistor, two input signals are combined into a single signal after respectively passing through the single-ended amplifier, so wideband frequency characteristics can be obtained while reducing current consumption.
However, the related art active balun having the stacked structure including the common drain MOS transistor and the common source MOS transistor has a problem in that the parasitic capacitances of the respective input transistors viewed from the single output are different, making the two input signals have different levels.